Currently, much industrial fermentation involves the manufacture of ethanol for either chemical or fuels use. For use in fuel, butanol has advantages as compared to ethanol, namely butanol has a lower vapor pressure.
An advantageous butanol fermentation process may encompass a complete, or substantially complete, conversion of sugars to butanol by a microorganism without reaching a butanol titer above a threshold of butanol tolerance of the microorganism that may cause the rate of butanol production to fall below an undesirable predetermined rate. While it may be possible to limit sugar loadings to a level whereby batch fermentation does not require operation at a butanol concentration above the tolerance level, this approach has disadvantages because limited sugar loadings may result in reduced capital productivity and dilute solutions that may be economically undesirable to process. Therefore, there is a need for a process by which levels of butanol are limited in a fermentation at or below the tolerance level while sugar loadings are not limited by considerations of the tolerance level.
One means by which a butanol-producing fermentation process might be made more efficient would be to remove butanol as it is being formed from the fermentation medium (or broth), for example, by a vaporization process, so that the tolerance level of the butanol-producing microorganism is not reached, allowing high loading of sugar to be charged to the fermentation vessel. Such an “in situ product removal” (“ISPR”) process is described, for example, in PCT International Publication No. WO2009/079362; U.S. Patent Application Publication No. 2012/0035398; and U.S. Patent Application Publication No. 2012/0211348.
As fermentation relies on microorganisms, any temperature constraints relative to the microorganisms may be considered. To operate a vaporization process at acceptable temperatures, consideration must be given to costs and practicalities of cooling and condensing vapor streams containing product alcohols, or operation under vacuum. The costs associated with removal of heat within a chemical process can be a function of the plant location and also the time of the year. In many geographic areas, it is not possible to guarantee cooling to be available or practical at the temperature at which heat needs to be removed from the vapor stream.
Providing chilled water to the heat exchanger by which condensation is carried out significantly increases the cost of the cooling medium. An alternative would be to compress the vapor stream to a slightly higher pressure to allow the condensation to be carried out at a higher temperature. Processes described which use lithium bromide and similar hygroscopic solutions for absorption of ethanol and water vapor may not be adequate for absorbing carbon dioxide or higher alcohols of a vapor stream.
In addition, with whatever method is used, there will be a residual gas stream (e.g., carbon dioxide in the fermentation broth) that must be compressed before discharge to the atmosphere. While vacuum flashing represents an effective means by which butanol can be removed from a fermentation process, there is a need for advances in the processing of the resulting low pressure vapor stream containing butanol or other product alcohols.